Current nomograms for diagnosing prostate cancer presence are unable to differentiate between indolent and aggressive disease resulting in a high number of unnecessary surgeries. The creation of improved diagnostic markers, such as circulating tumor progenitor cells, would improve patient treatment by identifying prostate cancer patients with aggressive disease who are more likely to progress to bone metastasis. Several markers of tumor progenitors, including CD117, have been identified based on their expression in primary tumors. Staining of CD117 and its ligand stem cell factor (SCF) are both upregulated with cancer severity, with the highest expression occurring in bone metastatic prostate cancer tumors. Preliminary experiments have shown that CD117 expression has also been found on circulating progenitor cells in prostate cancer patients and the levels of CD117+ cells in the circulation increased with cancer severity. The objective of this proposal is to determine how CD117 cells are mobilized into the circulation and contribute to prostate cancer metastasis. Using murine models, (1) the effects of C117 activation and downstream signaling on tumorigenicity and premetastatic niche formation will be examined and (2) the role of SCF expression originating from the microenvironment or the tumor itself on tumor growth, CD117+ cell mobilization and premetastatic niche formation. Experimental studies will utilize comparisons between CD117+ and negative prostate cancer cell populations tested in vivo and in vitro to measure changes in tumorigenicity, metastatic potential, and downstream signaling pathway activation. In addition, CD117+ cell localization and proliferation in vivo will be measured using two-photon imaging. Further, using transgenic mice with SCF labeled by GFP and cell-specific conditional knockout mice, the exact source of microenvironment-derived SCF and its effects on CD117+ and negative tumor cell tumorigenicity and mobilization will be measured. Correspondingly, alterations in tumor-derived SCF and their effects on CD117+ prostate cancer cell proliferation and metastatic potential will be examined. The long term objective of this proposal is to develop novel tools to differentiate patients likely to progress t metastasis and thus requiring aggressive treatment from those that would benefit from active surveillance. Understanding the mechanisms controlling metastatic initiation and premetastatic niche formation will lead to significant advances in the treatment and diagnosis of metastatic prostate cancer. More reliable identification of prostate cancer patients likely to progress to invasive or metastatic disease will result in earlier and more aggressive interventions in this patient population, while preventing excessive treatment of low grade cancer patients. Any findings and therapeutic developments discovered by the proposed research may extend to other tumor types with a tumor progenitor cell component (i.e. GIST, breast).